Moving charged particles may change their paths by an electrostatic field or a magnetic field, thus a charged particle path control apparatus that serves to control a propagation direction of a charged particle beam in which several charged particles move together, or focus or spread the charged particle beam, is called a charged particle optical system, which refers to an optical system controlling a path of light.
The charged particle beam includes an ion beam and an electron beam. The ion beam is mainly a positive ion beam, and extracted from a plasma ion source, a gas field ion source, or a liquid metal ion source, and the electron beam is obtained from a cathode. The electron beam is obtained from a schottky electron source in which zirconia oxide (ZrO) is coated on a tungsten (W) surface, or a cold field emission electron source including tungsten. In the ion beam and the electron beam, ions or electrons out of a predetermined energy range that ions or electrons forming a beam averagely have are present, and paths of such ions and electrons are out of a desired path range in the charged particle optical system controlled based on the predetermined energy range.
A monochromator selecting a particle of a specified energy range in the charged particle beam and removing a particle having energy out of the range, includes a Wien filter monochromator using an electric field and a magnetic field together, an electrostatic field monochromator making the charged particle beam to perform a circular movement in an electrostatic field, and a Mollenstedt energy analyzer monochromator in which propagating charged particle beam is incident on an asymmetric electrostatic field to move a path, or the like.
Here, the cylindrical lens is configured of three electrodes having a rectangular opening at the center thereof. A center electrode is applied with high voltage, two front and rear electrodes are charged particle lenses that is applied with ground voltage, and an insulation material is provided between the three electrodes. Energy of the charged particle is decelerated to almost 0 around the center electrode applied with high voltage, a component passing through an outside of an optical axis of the lens is selected, the energy of the charged particle is analyzed by using energy dispersion occurring by chromatic aberration of an axis of lens. In the monochromator, the off-axis component of the charged particle source is filtered by the energy selection aperture, energy is divided by passing light through the outside of the axis of the electrostatic lens to select only a mean energy part. The monochromator may be used in the charged particle beam apparatus such as a scanning electron microscope (SEM). The monochromator reduces an influence of chromatic aberration of the charged particle beam to thereby improve image resolving power.
Korean Patent Laid-Open Publication No. 10-2015-0146079 relates to a monochromator and a charged particle beam apparatus including the same, and discloses a technology for a monochromator including two electrostatic lenses each including a plurality of electrodes, and an energy selection aperture provided therebetween. However, in Korean Patent Laid-Open Publication No. 10-2015-0146079, a center of a rectangular opening of the two rectangular electrostatic lenses needs to be separately offset with respect to an optical axis of the charged particle beam, and the number of electrodes configuring the electrostatic lens is large, thus it is complicated to precisely match openings of the electrodes.